1. Field of the Invention
The present invention relates to a semiconductive rubber belt used in a transferring section or the like in an image forming apparatus based on electrostatic photography, such as a copying machine, a printer, or a facsimile machine; and a process for producing the same.
2. Description of Related Art
The semiconductive rubber belt used in a transferring section or the like in an image forming apparatus is generally a belt containing a conductive filler, such as graphite or carbon black, and having a volume resistivity adjusted to the range of 104 to 1012 ohm-centimeters (Ωcm). The range of the volume resistivity is generally in the so-called semiconductive range. In this range, the electric resistance of the semiconductive rubber belt tends to vary in accordance with a slight variation in the amount of the filler that is added to the belt. Then the electric resistance of the rubber belt is varied, formed images may become uneven. Thus, it is very important to make the electric resistance of the semiconductive rubber belt even as a whole. A process that is known for producing this semiconductive rubber belt, is the “tube extrusion molding process”, wherein an unvulcanized rubber composition into which a filler is blended is subjected to tube extrusion molding to form a tubular unvulcanized rubber belt molded body; this body is allowed so cover a mold, and then the body is heated to be vulcanized, thereby yielding the belt.
In the tube extrusion molding process, the flowing rate (extrusion rate) of the unvulcanized rubber composition, into which the filler is blended, can also be set to a low value; therefore, a shearing force that is received by the unvulcanized rubber composition can be made relatively small when the composition passes through the cap or the like of the extruder. However, when an attempt is made for making the electric resistance of the semiconductor rubber belt even in the semiconductive range, the electric resistance of the belt may be locally varied even by a slight shearing hysteresis at the time of the extrusion molding. In particular, in bridge moieties, the number of which is usually three or more, located in a die (cap) necessary for extruding the unvulcanized rubber composition into a tubular form, weld lines of the unvulcanized rubber itself are generated in the width direction of the belt (the extrusion direction of the belt). In the moieties, the thickness of the belt is made small, and further a variation in the electric resistance may be generated. Accordingly, ordinary manners of the tube extrusion molding process have a problem that weld lines are formed at intervals in the “belt circumferential direction” orthogonal to the width direction of the belt so that the electric resistance of the resultant semiconductive rubber belt is varied in the belt circumferential direction.
Japanese Patent No. 3414514 described below discloses an image forming apparatus wherein the control of the intensity of transferring bias makes it possible to keep the charge amount of its semiconductive rubber belt constant even when a change is caused in the electric resistance of the semiconductive rubber belt in the belt circumferential direction (that is, even when the electric resistance of the semiconductive rubber belt is varied in the belt circumferential direction). However, the apparatus described in this document needs a separate device for controlling the intensity of the transferring bias. When the rotating speed of the semiconductive rubber belt is made high to heighten the image formation processing speed of the image forming apparatus, the intensity of the transferring bias is not easily controlled. Accordingly, in the market, it is largely desired to make the electric resistance of any semiconductive rubber belt even at a high level.
Japanese Patent No. 3056413 described below describes a semiconductive belt in which on any single line along the width direction of the belt, the average of the molecular orientation angle θ to the belt width direction is set into the range of −15 to +15° and further the relationship between the average of the molecular orientation ratio correction value MOR-C of its conductive layer, the maximum value thereof, and the minimum value thereof is set to satisfy an expression of (the maximum value−the minimum value)/the average<0.4, whereby a variation in the electric resistance of the belt in the width direction thereof is decreased so that images to be formed can be prevented from becoming uneven. However, this semiconductive rubber belt cannot solve the problem that the electric resistance is varied in the belt circumferential direction since the belt is produced in an ordinary manner of tube extrusion molding process.
Japanese Patent. No. 3998344 described below discloses that according to the process for producing a semiconductive rubber belt using an extruder equipped with a crosshead having an inner cylinder section in which a spiral flow type groove is made, it is possible to produce a semiconductive rubber belt wherein the generation of a variation in the electric resistance is decreased. However, the present inventors have made investigations so as to make it evident that about semiconductive rubber belts produced by this producing process, there is a room for a further improvement, in particular, with respect to a variation in the electric resistance in the belt circumferential direction.    Patent Document 1: Japanese Patent Publication No. 3414514    Patent Document 2: Japanese Patent. Publication No. 3056413    Patent Document 3: Japanese Patent Publication No. 3998344